Often, fasteners used to assemble performance critical components are tightened to a specified torque level to introduce a “pretension” in the fastener. For example, high tensile-strength steel bolts used to fasten components of military vehicles, aerospace vehicles, heavy machinery, and equipment for petrochemical operations frequently have required torque specifications. As torque is applied to the head of the fastener, eventually, beyond a certain level of applied torque the fastener actually begins to stretch. This stretching results in pretension in the fastener which then holds the joint together. Overstressing fasteners can lead to their breakage whereas under-stressing bolts can lead to joint failure, leakage, etc. Furthermore, in situations where gaskets are being utilized between the components being joined, an unequally stressed set of fasteners can result in gasket distortion and subsequent problems like leakage. Accurate and reliable torque wrenches help insure that fasteners are tightened to the proper specifications.
Torque wrenches vary from simple mechanical types to sophisticated electronic types. There are several types of mechanical torque wrenches that are routinely used to tighten fasteners to specified torque levels. Of these, clicker type mechanical torque wrenches are very popular. Clicker type mechanical torque wrenches make an audible click to let the user know when a preset torque level has been achieved and simultaneously provide a feeling of sudden torque release to the user. One example of a clicker type torque wrench includes a hollow tube in which a spring and pawl mechanism is housed. The pawl is forced against one end of a bar that extends from a drive head. The bar and drive head are pinned to the hollow tube about a pivot joint and rotate relative thereto once the preset torque level is exceeded. The preset torque level is selected by a user by causing the spring to exert either greater or lesser force on the pawl. The force acts on the bar through the pawl to resist rotation of the bar relative to the hollow tube. As the torque exerted on the fastener exceeds the preset torque value, the force tending to cause the bar to pivot relative to the hollow tube exceeds the force preventing its rotation and the pawl “trips.” When released by the action of the pawl, the bar pivots and hits the inside of the tube, thereby producing a click sound and a sudden torque release that is detectable by the user. Typically, the preset torque values to assist the user in setting the torque wrench are permanently marked on a drum type scale that is visible through a window near or on the handle, or marked on the tube itself. For most clicker type torque wrenches, the preset torque is set by rotating either an adjuster sleeve on the handle, an end cap, or a thumb screw.
Another example of a clicker type torque wrench measures the deflection of a deflectable beam relative to a non-deflectable beam, the deflectable beam causing a click once the preset torque is reached. These and other types of clicker type mechanical torque wrenches are popular since they are relatively easy to operate and make torquing relatively quick and simple. The user merely sets the preset torque value and pulls on the handle until he/she hears and feels the click and torque release indicating to the user to cease torquing the fastener.
Several drawbacks limit the usage of clicker type torque wrenches. Often, these torque wrenches have permanently marked gages that are read by the user when setting the preset torque value. These gages can be hard to read, especially when the user is occupied with torquing a fastener with smooth and continuous motion to achieve proper fastening. Some existing torque wrenches address this issue by incorporating a magnifying glass or using a separate high resolution secondary scale. Still, the size of the markings is often small and the resolution of the markings is often limited by the physical space available on the gage. As well, the lack of high resolution may prevent the user from being able to preset to a desired torque value that includes a fraction of the desired units. Furthermore, these torque wrenches are often used in hard to reach, poorly lit areas, such as under the hood of an automobile, making readings potentially even more difficult.
As well, since the drum or other type of permanently marked gage can be fairly small, the upper torquing range of clicker type torque wrenches can be limited to less than the capability of the other mechanical parts of the wrench. Furthermore, in most prior art clicker type torque wrenches, the gages are marked with only one or two sets of units (i.e. foot-pounds and Newton-meters). The user is therefore limited to these two units and anything else is normally calculated manually.
Recalibration of existing clicker type torque wrenches, especially spring type clickers, often requires disassembling the unit to replace worn out parts, which can be expensive and time consuming. Recalibration is often needed to correct the effect of the spring's characteristics and mechanical wear that occurs over time. Often, such wear cannot be compensated for without recalibration since the gages are most often permanently printed on the handle.
The present invention recognizes and addresses the foregoing considerations, and others, of prior art constructions.